How Much For 12 Shirts?

I spotted this sign at a shirt shop while on holidays overseas:

Prices At A Shirt Shop - How Much For 12 Shirts
Prices At A Shirt Shop – How Much For 12 Shirts

One shirt costs $25. Two shirts costs $30. Seven shirts costs $100. Buy 10 shirts, get one free. How much should I have to pay for 12 shirts?